In the treatment of diseased conditions, therapeutic interventions are often undertaken which involve administration of foreign molecules having therapeutically beneficial effects. However, such administrations can often result in unwanted side effects resulting from activation of the body's immune response. Formation of antibodies following administration of therapeutics poses a serious clinical challenge. The antibodies can abrogate activity and/or alter pharmaco-kinetics of the therapeutic molecules.
Hemophilia A is an inherited bleeding disorder characterized by the deficiency or dysfunction of factor VIII (FVIII). FVIII serves as a critical cofactor in the intrinsic pathway of the coagulation cascade. Replacement therapy with recombinant human FVIII (rFVIII) or plasma-derived FVIII is the most common therapy employed in controlling bleeding episodes. However, the induction of neutralizing antibodies against the administered protein in approximately 15-30% of patients is a major complication in therapy [1-3]. The neutralizing antibodies frequently target the C2 domain, which is also involved in binding to phospholipids in vivo.
FVIII is a large multi-domain glycoprotein consisting of domains A1, A2, B, A3, C1 and C2. Systematic epitope mapping studies have revealed that anti-FVIII antibodies are mainly target defined regions in the A2 (heavy chain), A3 and C2 domains (light chain) of FVIII.
Another therapeutic molecule is Factor VIIa (FVIIa). This is a trypsin-like serine protease which plays an important role in activating the extrinsic coagulation cascade. FVIIa is a poorly catalytic form of factor VII after the activating cleavage between Arg152 and Ile153. Upon injury, circulating FVIIa becomes an efficient catalyst when forming a complex with tissue factor (TF), its allosteric regulator that is found on the outside of blood vessel. FVIIa-TF complex induces generation of small amounts of thrombin which further triggers blood clotting. Factor VIIa has been approved by the Food and Drug Administration in the United States for uncontrollable bleeding in hemophilia A and B patients who have developed inhibitory antibodies against replacement coagulation factors, factor VIII and factor IX. Intravenous administration of recombinant human Factor FVIIa (rHu-FVIIa) has been introduced because of fewer side effects than other alternative treatment strategies and to circumvent difficulty in preparing plasma-derived FVIIa. However, the short circulation half-life of FVIIa requiring repeated bolus injections to achieve desired efficacy can be problematic.
Additionally, other proteins involved in the blood coagulation cascade can also be used as therapeutics. Strategies that can inhibit processing by immune system and also prolong circulation time (reduce frequency of administration) would improve efficacy of therapeutic proteins. Therefore there is a need in the area of therapeutics to develop formulations that make the proteins less immunogenic, without affecting the circulating time or the efficacy.